This invention relates to digital type electronic time pieces, and more particularly to a motor drive circuit in such a digital type electronic time piece which drives a time indicating means with an electric motor in response to an input pulse signal.
One example of a conventional drive device in a digital type electronic time piece will be described with reference to FIG. 1.
A signal having a predetermined frequency outputted by a time reference source 1 utilizing a crystal oscillator, a commercial AC source, etc. is frequency-divided to 1/60 Hz, for instance, by a frequency division circuit 2, and is formed into a signal having a predetermined pulse width by a pulse width forming circuit 3. The output pulse signal of the pulse width forming circuit 3 is applied to a motor drive circuit 4 to drive an electric motor M. The motor M is a DC motor adapted to drive a time indicating means 5.
The pulse width of the pulse outputted by the pulse width forming circuit 3 is short, for instance several tens of milli-seconds to several hundreds of milli-seconds. Accordingly, in order to rotate the time indicating means a predetermined distance with this signal, it is necessary to provide a hold circuit in the motor drive circuit 4, which operates to hold the drive operation of the motor until the time indicating means 5 is rotated to cover the predetermined distance. Such a hold circuit has been disclosed by the applicant's Japanese Utility Model Application No. 32151/1975 (Japanese Utility Model Application Laid-Open No. 113464/1976). In this conventional hold circuit, the operation of a thyristor which is switched by the pulse from the pulse width forming circuit 3 or the like and the operation of a switch which is operated in association with the drive motor M of the time indicating means 5 are suitably combined to hold the drive of the motor. However, since the operation of the thyristor depends on temperature, erroneous operation may be caused in the hold circuit when ambient temperature changes occur. The conventional hold circuit is further disadvantageous in that setting the circuit constants in one attempt to eliminate this drawback is rather difficult. Furthermore, as holding current for holding the thyristor is necessary, the current consumption of the circuit is larger.
Another type of holding circuit is disclosed in U.S. Pat. No. 3,971,204. In that circuit, the drive signal is applied to the motor upon the occurrence of a pulse and the holding circuit is self-biased into a holding state until turned off by a stop signal supplied from a switch operated in conjunction with a predetermined degree of motor rotation. However, this circuit is still disadvantageous in that power consumption is high due to the self-biasing current which must be supplied.